From: sfjro@users.sourceforge.net
Subject: Re: [RFC PATCH 0/5] Union Mount: A Directory listing approach with lseek support 
To: bharata@linux.vnet.ibm.com
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
       Jan Blunck <jblunck@suse.de>, Erez Zadok <ezk@cs.sunysb.edu>,
       viro@zeniv.linux.org.uk, Christoph Hellwig <hch@lst.de>,
       Dave Hansen <haveblue@us.ibm.com>
In-Reply-To: <20071205143718.GC2471@in.ibm.com>
References: <20071205143718.GC2471@in.ibm.com>
Date: Fri, 07 Dec 2007 10:48:07 +0900
Message-Id: <E1J0SJz-0000w3-Cz@jroun>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1611
Lines: 38


Bharata B Rao:
> - The cache can grow arbitrarily large in size for big directories thereby
> consuming lots of memory. Pruning individual cache entries is out of question
> as entire cache is needed for subsequent readdirs for duplicate elimination.

Additionally, the memory usage may be a problem too since your
implementation calls kmalloc() for every names.


> - Whenever _any_ directory that is part of the union gets
> modified (addition/deletion of entries), the dirent cache of all the unions
> which this directory is part of, needs to be purged and rebuilt. This is
> expensive not only due to re-reads of dirents but also because
> readdir(2)/getdents(2) needs to be synchronized with other operations
> like mkdir/mknod/link/unlink etc.

The cache in struct file doesn't need to be refreshed unless rewinddir()
is issued. Also you can maintain the cache in every add/del entries,
instead of discarding the cache entirely.


> After all this, I am beginning to think if it would be better to delegate
> this readdir and whiteout processing to userspace. Can this be better handled

Yes, I had such idea once. And copy-up too. They can be done in
userspace (while you need to be careful about the privilege).

Anyway I agree with you. As I wrote before, this approach consumes a lot
of memory and cpu (for comparing whiteouted names).


Junjiro Okajima
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